A conventional technique of the type described above is shown in FIG. 1.
In FIG. 1, an electrode (1) for machining a workpiece (3) with a configuration confronts the workpiece (3) disposed within a machining tank (2) through a machining gap (G). The tank (2), and consequently the gap (G), are filled with an insulating machining solution (4). A machining power source (5) includes a DC power source (51), a switching element (52) for switching the machining power source (5), a current limiting resistor (53) and an oscillator (54) for controlling the switching operation of the switching element, and supplies a switching current (I) between the electrode (1) and the workpiece (3). The switching current (I) is represented by the expression I=(.sup.E-V g)/R, and the interelectrode voltage (V.sub.g) becomes 20 to 30 V during arc discharge, 0 V at short-circuiting time and E during no discharge. The interelectrode voltage V.sub.g becomes 0 V when the switching element (52) is in its off state. If the interelectrode voltage V.sub.g is detected and is averaged by a smoothing circuit (6), the machining gap can be controlled by the averaged value of the interelectrode voltage V.sub.g. More specifically, when the machining gap is wide, a discharge will hardly occur so that the average voltage (VS) becomes high. When the gap is narrow, a short-circuit will occur between the workpiece and the electrode to be readily discharged, causing a reduction in the average voltage. Accordingly, when the voltage (VS) is compared with a reference voltage (Vr) and the difference voltage therebetween is inputted to an interelectrode servo circuit in which the difference voltage is amplified by an amplifier (7) and amplified difference voltage is inputted to a hydraulic servo coil (8), a hydraulic servo mechanism which includes a hydraulic pressure generating pump (9), a hydraulic cylinder (10) and an electrode supporting rod (11), and so forth, is so controlled that the interelectrode gap (G) may substantially become constant. The interelectrode servo circuit and the hydraulic servo mechanism form electrode feeding means.
The most general measure for determining the propriety of the machining conditions, or interelectrode state by the conventional electric discharge machining apparatus is to observe the average interelectrode voltage V.sub.g. If the average interelectrode voltage V.sub.g is low, the interelectrode impedance is low. This is considered to cause a short-circuit between the electrode and the workpiece, a continuous arc discharge, machining powder or chips between the electrodes, and sludge accumulated therebetween, and so forth. Since carbon is generated due to the thermal decomposition of the machining solution if the most dangerous abnormal electric arc discharge has once occurred in an electric arc discharge machining, an electric discharge occurs between the carbon and the workpiece. This increases the interelectrode impedance, with the result that the detection of the deterioration in the interelectrode state due to the average voltage becomes impossible.
Another conventional method of detecting abnormal electric arc discharge between electrodes includes the steps of observing the operation of the electrodes with a mechanical gauge such as a dial gauge, and determining the electrode vibrating state and the stability of machining the workpiece at the machining time. This method requires observation in the vicinity of the electric discharge machining apparatus. It is accordingly impossible to detect the deterioration of the interelectrode state during electrical discharge from a location apart from the electric discharge machining apparatus during an operatorless operation.
It is evident from the foregoing description that the state of the interelectrode can be readily determined if the output detected by the aforementioned dial gauge is converted into an electric signal, snce the electric signal like the above average voltage can be observed at a place considerably isolated from the electric discharge machining apparatus. However, in the case where a digital numerical signal corresponding to the position of the electrode is observed, the signal will include a variety of superimposed numerical values, but does not contain data which can be separately identified as corresponding to the vibrating state. It is impossible and useless to read infinitesimal variations in an analog signal having a resolution of several microns, due to the fact that, if a total stroke of several hundred millimeters is to be observed, with 1 micron, converted into 1 mV, the total stroke corresponds to several hundred V, and further microminiature variations are to be read from the observed voltage.
The advantage of the dial gauge is such that, in the case of a dial gauge having 1 mm per one revolution, the displacement of 10 .mu.m can be observed as the variation of 3.6.degree. and the total stroke is covered by rotating the gauge many times. It was, however, heretofore impossible to convert the function into an electric measure so as to observe it at a place apart from the apparatus and to suitably identify the occurrence of abnormality between the electrodes.